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LaPlace’s Demon and Quantum Aggregates

by Bill Meacham on December 5th, 2014

With the notable exception of chaotic systems—those in which slight variations of initial conditions produce widely diverging outcomes(1)—the theory that everything is determined generally entails that future states can be predicted from current or past states of the system under investigation. The possibility of accurate prediction has a distinct bearing on questions of determinism and free will. For the most part, if something cannot be predicted with accuracy, then it is not determined.

Materialists base deterministic beliefs on physical causality, the idea that physical events happen inexorably as a result of prior physical events. Taking human beings to be nothing more than complex aggregations of physical matter, they believe that our sense of free will is illusory, and that all is determined by the past. If we insist that such a view entails that we could fully predict the future, we run into a problem. For any system that engages in substantial interaction with its environment and is complex enough to be interesting, it would be computationally unworkable to predict its future states in their entirety. We might get better and better, of course, but could not achieve 100 percent accuracy. Even disregarding quantum indeterminacy, it is in practice impossible fully to predict the future.

Even so, some insist that it could be possible in principle. If we had a powerful enough computer and enough data, they say, we could do it. This was the view of the Marquis de LaPlace, who wrote,

We may regard the present state of the universe as the effect of its past and the cause of its future. An intellect which at a certain moment would know all forces that set nature in motion, and all positions of all items of which nature is composed, if this intellect were also vast enough to submit these data to analysis, it would embrace in a single formula the movements of the greatest bodies of the universe and those of the tiniest atom; for such an intellect nothing would be uncertain and the future just like the past would be present before its eyes.(2)

LaPlace knew nothing of computers, but LaPlace’s Demon (so-called; he himself did not use that term(3)) takes the place of one. The problem is that, given the openness of systems to external influences, such a computation would mean ultimately that we would need to predict the future of the entire universe. To do so would require a computer with a data store larger than all the items we would need to keep track of, hence larger than the universe. Not to mention that the computer itself would presumably be part of the universe and thus would itself need to be modeled. This scenario ends up in absurdity.

At the quantum level the future state of an individual object or event (at that level, the distinction between the two is tenuous at best) is indeterminate; events can be predicted only statistically. However, the statistical predictions are quite accurate and replicable. This leads some materialists, who believe humans to be entirely physical, to assert that human beings are determined because we can predict physical reality with accuracy. This does not hold either. It is the same as saying that people are determined because given a population of them we can predict how many will choose one thing over another—to vote Republican or Democrat, say. Or that an individual is determined because over a span of time we can predict how often that person will choose one thing over another—to eat vanilla ice cream or chocolate, for instance. But even given the accuracy of such statistical predictions, we are unable to predict a single instance with certainty. We can’t fully predict how a particular person will vote or what food that person will choose at a particular time.

The single instance of person or time is analogous to the single photon fired at the photographic plate in the Double Slit experiment.(4) We are unable to predict where it will be detected, even though we can predict the statistical aggregate quite nicely. And that is the essential point about ourselves as agents, that in every moment there is the possibility that we will do something unexpected.


(1) Wikipedia, “Chaos theory.”

(2) LaPlace, A Philosophical Essay on Probabilities, p.4.

(3) Wikipedia, “LaPlace’s demon.”

(4) Wikipedia, “Double-slit experiment.”


Laplace, Pierre Simon. A Philosophical Essay on Probabilities. translated into English from the original French 6th ed. by F.W. Truscott and F.L., Emory. New York: Dover Publications, 1951.

Wikipedia, “Chaos theory.” Online publication as of 21 November 2014.

Wikipedia. “Double-slit experiment.” Online publication as of 16 September 2014.

Wikipedia. “”Laplace’s demon.” Online publication’s_demon as of 19 August 2014.

From → Philosophy

One Comment
  1. Carl Ehlert permalink


    It strikes me that you are mixing two very different arguments. One is my ability to predict any future event and two is the notion that all events are determined. My ability even with a super super computer to predict a future event is totally separate from the reality of determinism. If a drunk individual gets into and drives an automobile I may predict with varying degrees of accuracy that he will or will not be involved in an accident. I may also predict with varying degrees of accuracy that he, as a drunk driver is the cause of that accident if and when it might occur. The reality that he makes it home safely or does in fact have an accident is in no way dependent on my prediction. However, the facts preceding his safe arrival or his accident are totally dependent and determined by the events leading up to that point in time where he has an accident or pulls safely into his driveway and falls asleep. To say that all events are determined in no way implies that I have the ability to predict anything about the future. In fact since my prediction itself is determined by preceding events I might even say that my degree of accuracy in my prediction is also predetermined. To presume that any prediction I might make is a free choice is to assume in advance that free will is a reality and assumes the position that you are trying to prove.
    Even after the fact when we are trying to reconstruct the cause of an event be it a supernova or a car accident our ability to get it right is determined by how much and what kind of information we have at hand. We have moved from a position where all happenings are the result of the will of some unseen god to a time when we recognize that there are natural forces at work independent of any sentient entity.
    The primary purpose of so called “free will” is the human need to assign responsibility, assess blame and most importantly to inflict punishment. I suspect that if we live long enough as a species the day will come when we will recognize that while many factors go into determining why we act as we do ultimately we have no actual control. We will then begin the slow and arduous task of attempting to learn which factors produce which qualities ( a process that has already begun with designer babies and genetic manipulation) and assume that we can yet again predict or circumvent our determined world by that mystical quality we call “free will”.
    As you might expect I am not optimistic either about the longevity of the human species or about our ability to create the “perfect” human being.
    I suspect that the religious among us probably have it right when they say “God’s will be done!” except that God is not some super being but simply our deterministic universe in action.

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